Conventionally, in an image forming apparatus such as a laser printer, a developing cartridge containing toner has been detachably mounted on an apparatus body. The developing cartridge is provided with a toner containing part that contains the toner, a supply roller for supplying the toner contained in the toner containing part, a developing roller which carries the toner supplied from the supply roller, and a layer thickness regulating blade which is brought into pressure contact with the developing roller.
When the layer thickness regulating blade is in pressure contact with the developing roller, a thin layer of the toner is formed on a surface of the developing roller. When the developing cartridge is opposed to a photosensitive drum which is mounted on the apparatus body, the toner carried in a center area of the developing roller is used for developing an electrostatic latent image which has been formed on the photosensitive drum. However, the toner carried in side end areas of the developing roller will stay there, and an amount of the toner will be increased. Consequently, the toner will likely leak from end parts of the developing roller.
In order to solve the above described problem, it has been proposed in JP-A-2003-195628, for example, that in a layer thickness regulating blade formed of a leaf spring which is provided with a pressing part of silicone rubber, both end portions corresponding to side end areas of the developing roller are cut away, and the silicone rubber in these portions is formed in a discontinuous shape having a different sectional shape from the center portion.
Specifically, in a case where the pressing part of the layer thickness regulating blade has a curved face in an area to be in contact with the developing roller, a radius of curvature in both end portions of the pressing part is made smaller than in the center portion. On the other hand, in a case where the pressing part in an area to be in contact with the developing roller has a flat face, an angle of both end portions of the pressing part with respect to a tangent of the developing roller is made larger than that of the center portion. In this manner, the pressing part is formed so that a larger amount of the toner can be scraped off in both the end portions of the pressing part.
However, because the pressing part of the above described layer thickness regulating blade is formed of silicone rubber, the silicone rubber will be worn by sliding friction with the developing roller, when printing has been repeated. Particularly, both the end portions of the pressing part are formed in such a shape that a larger amount of the toner can be scraped off the end portions than the center portion, and will be worn to a larger extent, as compared with the center portion. When both the end portions of the layer thickness regulating blade have been worn from longtime use, in this manner, the toner in the side end areas of the developing roller cannot be sufficiently scraped off, and will stay in these areas. As the results, there is a problem that leakage of the toner from the end parts of the developing roller may occur.